Nonstereospecific transamination catalyzed by pyridoxal phosphate-dependent amino acid racemases of broad substrate specificity.
Pyridoxal 5'-phosphate-dependent amino acid racemases of broad substrate specificity catalyze transamination as a side reaction. We studied the stereospecificities for hydrogen abstraction from C-4' of the bound pyridoxamine 5'-phosphate during transamination from pyridoxamine 5'-phosphate to pyruvate catalyzed by three amino acid racemases of broad substrate specificity. When the enzymes were incubated with (4'S)- or (4'R)-[4'-3H]pyridoxamine 5'-phosphate in the presence of pyruvate, tritium was released into the solvent from both pyridoxamine 5'-phosphates. Thus, these enzymes abstract a hydrogen nonstereospecifically from C-4' of the coenzyme in contrast to the other pyridoxal 5'-phosphate-dependent enzymes so far studied, which catalyze the stereospecific hydrogen removal. Amino acid racemase of broad substrate specificity from Pseudomonas putida produced D- and L-glutamate from alpha-ketoglutarate through the transamination with L-ornithine. Because glutamate does not serve as a substrate for racemization, the enzyme catalyzed the nonstereospecific overall transamination between L-ornithine and alpha-ketoglutarate. The cleavage and formation of the C-H bond at C-4' of the coenzyme and C-2 of the substrate thus occurs nonstereospecifically on both sides of the plane of the coenzyme-substrate complex intermediate. Amino acid racemase of broad substrate specificity is the first example of a pyridoxal enzyme catalyzing nonstereospecific transamination.
Lim
Y H
YH
Laboratory of Microbial Biochemistry, Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan.
Yoshimura
T
T
Kurokawa
Y
Y
Esaki
N
N
Soda
K
K
D000591:Amino Acid Isomerases / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism; D002384:Catalysis; D006859:Hydrogen / Q000378:metabolism; D007656:Ketoglutaric Acids / Q000378:metabolism; D008968:Molecular Conformation; D009952:Ornithine / Q000378:metabolism; D016958:Pseudomonas putida / Q000201:enzymology; D011732:Pyridoxal Phosphate / Q000494:pharmacology*; D011733:Pyridoxamine / Q000031:analogs & derivatives / Q000378:metabolism; D019289:Pyruvic Acid / Q000378:metabolism; D013379:Substrate Specificity; D000637:Transaminases / Q000378:metabolism*
D001426:Bacterial Proteins; D007656:Ketoglutaric Acids; D011732:Pyridoxal Phosphate; D011733:Pyridoxamine; D006859:Hydrogen; D019289:Pyruvic Acid; D009952:Ornithine; D000637:Transaminases; D000591:Amino Acid Isomerases; C010627:pyridoxamine phosphate